Picture an astronaut on Mars, shovel in hand, digging for water. How deep do they need to go? That single question is driving two new research papers from the Planetary Science Institute (PSI) in Tucson, Arizona. Led by scientists Hanna Sizemore and Samuel Courville, the studies produce fresh global maps of subsurface water ice on Mars — and, crucially, attach a statistical confidence score to every patch of ground they examine.

Here is the strange physics at the heart of this search. Mars has an atmosphere so thin that ice cannot simply sit on the surface; it skips straight from solid to gas in a process called sublimation. Bury the ice under even a thin layer of soil, however, and it survives. NASA's Phoenix lander once scooped up ice within centimetres of the surface near Mars's north pole.

Closer to the equator — exactly where future missions would prefer to land, because solar panels work far better there — the ice is expected to be buried much deeper, perhaps more than a metre down. The SWIM team, as the PSI-led Mars Subsurface Water Ice Mapping project is known, is using thermal data from two NASA orbiters to find that hidden ice. The key insight is elegant: a patch of ground hiding ice beneath dry sand cools and warms at a different rate than pure sand, pure rock, or pure dust does over the course of a Martian day and year. By analysing those temperature rhythms from orbit, researchers can effectively see underground.